A positive regulatory sequence of hepatitis B viral small X promoter

Hepatitis B viral X protein (HBx) and small X proteins (HBSx) are known to transactivate promoters for RNA polymerase II and RNA polymerase III. Small X promoter has been mapped in the 5'-distal half of the X open reading frame. A 5'-serial deletion analysis showed that there was a positive regulatory sequence for the efficient transcription of the small X promoter. Two cellular proteins of 110 kDa (p110) and 33 kDa (p33) bound at the 3' and 5' regions of the regulatory sequence, respectively. Mutation of a p33-binding and p110-binding sites led to diminution and elevation, respectively, of activation properties of the positive element, suggesting that p33 participates in the transactivation and that p110 has an inhibitory effect on the function of p33. This possibility was further supported by the result demonstrating that in vitro phosphorylation of p110 reduced its target DNA-binding capability.